Fiber Bragg gratings (FBG's) are portions of optical waveguides, such as optical fibers, which have been processed to reflect and transmit specific wavelengths. The waveguides are typically germanium-doped silica fibers and for the purposes of this description will be referred to as “fibers” or “optical fibers.” However, it should be understood that these terms are being used in a generic sense to mean any type of optical waveguide.
Producing FBG's involves exposing the fiber to ultraviolet light, the intensity of which varies between light and dark along the length of the fiber. The light and dark bands of exposure are placed along the fiber with spacing comparable to the wavelength of light to be reflected by the fiber in operation. The ultraviolet light induces changes in the index of refraction of the fiber, producing an index grating along the length of the fiber.
A light source used for exposure of a fiber to make FBG's must provide light within specific ranges of wavelengths in the ultraviolet portion of the spectrum. A typical fiber's primary wavelength range for absorption peaks near 240 nm, and wavelengths differing from the peak by more than about 10 nm are significantly less effective. Even at the peak wavelength, only a small fraction of the laser power is absorbed, so it is highly desirable for the light source to provide light at a wavelength near the absorption peak.
Unfortunately, the current sources of ultraviolet light used for FBG production have various drawbacks. Most current production systems for FBG's use either KrF excimer lasers emitting at 248 nm, or frequency-doubled argon-ion lasers at 244 nm, to expose the fibers. KrF excimer lasers can produce high average powers, which facilitates processing, but they have serious disadvantages. They require toxic, corrosive gases for operation, have high operating and maintenance costs, and produce short duration (˜50 ns), low repetition rate (<1000 Hz), high peak output power (˜1 MW) ultraviolet pulses. The high peak output powers cause damage to the fibers, weakening them and making them susceptible to fracture. The alternative continuous wave (“cw”) argon ion lasers suffer high operating costs and produce weak output powers of one-half watt or less, leading to poor production throughput.
Several other types of lasers, including argon-fluoride and xenon-chloride excimer lasers and flashlamp-pumped lasers, have also been applied to FBG production. However, the argon-fluoride and xenon-chloride excimer lasers suffer from disadvantages similar to those of KrF excimer lasers. Flashlamp-pumped lasers provide some operational improvements compared to excimer lasers, but also generate high peak power pulses which damage the optical fibers.
Other lasers have also been used in laboratory demonstrations of FBG production. Frequency-doubled copper vapor lasers at 255 nm have been used, though their output wavelength is slightly too long to be optimal. Frequency-doubled liquid dye lasers have been tuned to the 240 nm region for FBG fabrication, but such lasers are impractical for large-scale industrial production, since they require very frequent changes of the liquid dye solution to maintain operation.
Solid-state lasers are being increasingly utilized for materials processing applications, due to their reliability and reasonable operating costs. Solid state lasers would be of great interest for FBG production, but heretofore have not been usable because they have not been able to produce the required wavelengths.